Admittedly I only used a cleartext password for a couple of my menu items, but this is from the syslinux menu.txt documentation and should help:
Code:
MENU PASSWD passwd
(Only valid after a LABEL statement.)
Sets a password on this menu entry. "passwd" can be either a
cleartext password or a password encrypted with one of the
following algorithms:
MD5 (Signature: $1$)
SHA-1 (Signature: $4$)
SHA-2-256 (Signature: $5$)
SHA-2-512 (Signature: $6$)
Use the included Perl scripts "sha1pass" or "md5pass" to
encrypt passwords. MD5 passwords are compatible with most
Unix password file utilities; SHA-1 passwords are probably
unique to Syslinux; SHA-2 passwords are compatible with very
recent Linux distributions. Obviously, if you don't encrypt
your passwords they will not be very secure at all.
If you are using passwords, you want to make sure you also use
the settings "NOESCAPE 1", "PROMPT 0", and either set
"ALLOWOPTIONS 0" or use a master password (see below.)
If passwd is an empty string, this menu entry can only be
unlocked with the master password.